1. Field of Invention
Known methods of thermally curing coatings in laboratories and production facilities, include:                a.) heating of coatings via hot air convection ovens, which may require 25-45 minutes for full curing;        b.) infrared pre-heating coatings prior to convection heating, which reduces the curing time; and,        c.) infrared heating coatings for the total curing process which offers significant reduction of curing time, see Applicant's co-pending application Ser. No. 09/843,967, filed in Apr. 27, 2001 disclosing an in-line curing operation.        
The above-methods include the coating curing process of:                a.) powder coat processing where the powder is heated until it gels and held at a curing temperature for, in most materials, cross polymerization; and,        b.) wet coat processing where fluid is driven from the coating and held at a curing temperature for thermal setting or cross polymerization.        
There are numerous types of coating materials, all of which require specific curing temperature vs. time profiles when processing wet coatings. Also, specific gels and processing temperature vs. time profiles are required when processing powder-coating products. Normally, specific process details for establishing production finishing systems having appropriately designed features are not available to the finishing system designers. This can potentially cause over or under design of the required production facility.
Known methods of curing coatings via ultraviolet energy sources include preheating the powder coating to a gel temperature prior to being exposed to the ultraviolet energy source. This process typically utilizes an infrared energy source operating in the medium wave length range, which permits energy to be transferred into the coating via radiant energy penetration principles. This method offers significantly improved energy transfer rates. Using medium wavelength energy, normally does not allow penetration deep enough to directly introduce heat into a substrate material on which this coating is placed. The substrate material will in turn draw energy from the coating. The thicker the substrate, the greater the energy drawing magnitude; thus, creating large temperature gradients across the coating thickness and extending the processing time. Energy is drawn from the coating, which delays a wetting action between the coating and substrate. This energy drawing of the substrate from the coating also increases the temperature gradients across the coating thickness which decreases uniform cross linking and/or curing of the coating material. The greater the temperature gradients across the coating, the greater the time lag of the bonding process between the coating and the substrate. The higher the temperature of the substrate, the better the wetting action and thus, the better the bonding action at a coating and substrate interface.
The process may utilize conventional hot air convection or long wavelength infrared heating sources to apply energy to the surface molecules of the coating (no radiation penetration) in this case, the energy is transferred inward by conduction principles. These methods require substantial time and create significant undesired temperature gradients across the coating thickness. These sources of energy require significantly longer times to process than the afore-mentioned medium wavelength infrared source.
It is also known to use short wavelength energy sources as a means of curing coatings. When short wavelength energy sources are utilized, they offer the ability to heat the coating very rapidly, and, in most coatings materials, penetrate through the coating. Most of the energy is deposited in the coating; however, a substantial amount of energy is still delivered into the substrate. The amount of energy introduced into the substrate is affected by the coating material's formulation and thickness. This introduction of energy into the substrate reduces the temperature gradient across the coating thickness, and enhances initiation of the on-set of polymerization or curing, as well as, bonding at the coating and substrate interface.
Even though the short wavelength energy source has excellent capabilities, there are inherent features in certain applications, which create limiting processing conditions. For example, when the energy needs to be reduced in an appropriately designed curing system because the conveying mechanism velocity must be reduced, the normal response is to reduce the voltage in order to reduce energy. The wavelength of the energy source may be reduced to provide a temperature where its wavelength may be in the medium or long wavelength energy spectrum, which diminishes the unique features of short wavelength energy. Examples of these limitations would be one where a curing oven, utilizing short wavelength sources, was appropriately designed for a given production velocity and a need to reduce the velocity develops due to a production limitation. In this case, the energy wavelength would be greatly changed, causing process deficiencies due to inadequate energy penetration.
Normally, when curing coatings with ultraviolet energy sources, the coatings are thermally processed before exposure to the ultraviolet source. The pre-heating methods and their inadequacies are noted above. Ultraviolet energy sources have discrete energy radiation peaks. Critical production and specification requirements can arise from the paint manufacturer. Their enhancing light sensitive additives must function at discrete frequencies of the process ultraviolet energy source to attain adequate curing.
There are numerous types of coating materials, all of which require specific pre-heat temperature/time profiles and ultraviolet energy spectrum data. The lack of laboratory test information and its coordination with production performances sometimes leads to the construction of production facilities being under or over designed.
Laboratory pilot test systems typically consist of a conveyorized arrangement, which requires significant floor space. They normally utilize a single type of thermal source with minimal process controls. These systems include heating methods with inadequacies as defined above. They likewise include minimal programming and recording instrumentation.